A conventional metal-matrix composite may be incorporated with reinforcing material therein for strengthening its mechanical property.
According to Orowan's theory, when a dislocation movement in the metal matrix encounters a second-phase or dispersion reinforcing material, the dislocation movement will be hindered so as to prevent material deformation and to strengthen the properties of the composite.
Reviewing a particle dispersion theory as disclosed by Orowan, a strength increment (.tau.-.tau..sub.0) of a metal-matrix composite may be expressed in the following formula: EQU .tau.-.tau..sub.0 =Gb/.lambda.
wherein, .tau. and .tau..sub.0 are each a yeild strength of the metal-matrix composite and its original metal matrix, G is a shear modulus of the metal matrix, b is a Burgers vector, and .lambda. is a distance between two neighbouring particles of the second-phase or dispersion reinforcing material.
Assuming the particle of the reinforcing material is spherically shaped, the distance .lambda. between particles of the reinforcing material will be: EQU .lambda.=4(1-f)r/3f
Where f is a volume fraction of the reinforcing material particle in the matrix, and r is a radius of the reinforcing material particle. material particle.
Then, the strength increment of the metal-matrix composite is formulated to have to do with the radius of the particle of the dispersion reinforcing material as follows: EQU .tau.-.tau..sub.0 =3fGb/4(1-f)r
For the above-mentioned formula, if the volume fraction f of the reinforcing material particle in the matrix is fixed, the strength increment (.tau.-.tau..sub.0) of the metal-matrix composite will be increased depending upon a decrease of particle radius r of the dispersion reinforcing material.
Therefore, it is expected to make the reinforcing material as fine as possible and expected to homogeneously disperse the reinforcing material in the matrix to thereby enhance the mechanical properties of the metal-matrix composites.
However, the fine reinforcing material, once directly fed into the metal matrix, the fine particles due to Van der walls force existing among the particles, will cluster in the metal matrix mixed with the fine particles of the reinforcing material, thereby causing unhomogeneous dispersion of the fine particles in the matrix and deteriorating the property of a finished casting product therefrom.
Meanwhile, a fine reinforcing material of dry particulates is directly incorporated into a molten metal alloy such as an aluminum alloy, the dry fine particulate reinforcing material will easily fly over as effected by a convection hot air streamflow above the molten alloy to cause loss of the fed reinforcing material. Meanwhile, the feed rate for adding the fine reinforcing material into the matrix will be difficultly controlled.
By using a vortex agitator for refining a metal-matrix composite, gases may be directed into the molten metal solution which should be removed by a degassing operation before casting process. Re-melting the metal-matrix composite under high vacuum degree may remove partial gases in the composite. However, the molten metal solution has a high viscosity, thereby being uneasy to extract gases outwardly from the viscous molten solution.
It is therefore expected to invent a process for well incorporating fine particulate reinforcing material into the metal matrix during its refining process, and also providing a reliable degassing operation for efficiently removing gases in the composite product.